The term polymer composite generally refers to materials produced by impregnating fibrous materials with resins that are used in a variety of industries, ranging from aerospace, motor sports, automotive, boating, civil construction and are generally formed in laminates or layers. The fibres used in such composites are varied and include carbon, aramid (Kevlar) and glass and the resins are generally selected from thermoplastic or thermosetting resins such as epoxy, cyanate, phenolic and the like.
The thermoset resin impregnated fibres are commonly soft, flexible, slightly tacky materials, comprising unidirectional fibres or woven cloth. The components of a polymer composite must be formed or cured under conditions of elevated temperature and pressure to compress the material and form the polymer composite.
In the case of a thermoplastic composite, the addition of heat serves to enable the flow of a thermoplastic resin which is solid at room temperature. The combination of pressure and temperature enables the molten resin to impregnate the fibres and form the thermoplastic polymer composite to the desired shape. The thermoplastic polymer composite then solidifies upon cooling to form the desired part.
Composite structures made from laminates and composite sandwich structures made from laminates and lightweight cores (honeycombs, foams, wood etc.) can suffer numerous defects as a direct result of the manufacturing process. Common problems include resin exothermic reactions, mould surface pitting, inhibited resin flow, internal porosity, voids, and a poor skin to core adhesion.
Voids and porosity are considered to be the most serious defects to deteriorate the mechanical properties of thermosetting polymer composites. Voids are the result of the expansion of volatiles in gaseous state during the cure cycle. Moisture is the major source of volatiles found in pre-impregnated thermosetting composite materials (pre-pregs). Pre-pregs readily absorb moisture from the surrounding atmosphere, with moisture content a function of the relative humidity, ambient temperature and pre-preg resin content. As the temperature of a thermosetting polymer composite component increases during the cure cycle, the vapour pressure from these volatiles increases. These pockets of gas are trapped in the polymer composite as the resin gels, forming spherical voids that can act as initiation sites for fatigue or fracture.
Porosity is a similar phenomenon to voids and is a result of air trapped between the plies of pre-preg during lay-up. These air pockets cannot be removed by simple vacuum consolidation and remain locked into the resin matrix during cure.
Current industry curing and forming methods involve ovens or autoclaves, which are pressurised ovens. Through the application of pressure, many of the above problems are minimised. However, some problems are not and several more are introduced as a direct result of the autoclaving process.
Thermosetting resin curing reactions are exothermic and the heat generated during curing creates serious problems (such as composite ignition) unless it is dissipated to the surroundings. The relatively low specific heat of the air or nitrogen, as used in autoclaves, ensures that little energy is absorbed by the gases during curing of the composite. Consequently, the autoclave temperature must be ramped at a slow rate, typically 1-3° C. per minute, up to anywhere between 100° C. and 200° C. Slow ramping ensures the temperature difference between the composite and the surrounding gas is enough to safely absorb the heat. The slow ramp rate requirement provides long cycle times, which can be inefficient for large volume part production. The slow ramp rates required are also a by-product of the poor thermal conductivity of gases. Raising the temperature too fast can create thermal gradients across the autoclave chamber if the heat sources are few in number and unevenly dispersed. This thermal gradient may produce thermal stresses in the polymer composite component as one end cures faster than the other.
Thermoplastic polymer composite forming methods require the input of heat energy to raise the temperature of the thermoplastic composite to a point at which the thermoplastic resin melts and flows. Typically, this is completed in an oven where the limitations imposed by the heat transfer rate of a gas or air may cause long ramping periods. Alternatively, the process is completed in an extruding machine which enables rapid ramp rates due to contact between the thermoplastic composite and metallic heating elements. However, the use of extrusion machines in the generation of thermoplastic polymer composites is extremely limited as the act of extruding, in which the resin is forced through a small heated opening, limits the quantity of fibre reinforcement that may be incorporated into the part. This quantity of reinforcement distinguishes between what is commonly regarded as a thermoplastic part and a thermoplastic polymer composite part.
A typical cure cycle in an autoclave involves a slow ramp up to the final cure temperature. As the temperature of the component increases, the resin viscosity is proportionately reduced. The reduced viscosity allows for increased gas flow, leading to the removal of entrained air and volatiles. This process continues until the cure reaction of the component begins to increase the viscosity faster than the increasing temperature can reduce it. Just before the reaction begins, the autoclave process holds the temperature briefly and increases the autoclave pressure to consolidate any remaining volatiles back into solution. Shortly after this, the reaction generates “gel” state in which the component becomes an amorphous semi-solid and no more gases can be removed. Optimisation of these fundamental process parameters allow for void levels to be reduced. This method is hampered however by the slow ramp rates required to avoid an exothermic reaction and the high pressures required to condense remaining volatiles. Furthermore, the volatiles that are condensed back into the composite during the pressure increase phase of the cycle detrimentally affect the polymer composite component by reducing the mechanical properties of the polymer component when exposed to elevated temperatures.
The autoclave process, in attempting to solve the problem of voids, introduces significant problems of its own. Apart from being capitally expensive, the high autoclave pressures can crush the core of a sandwich structure. The high pressures also require large pressure vessels to be manufactured, which are costly to purchase and maintain and require significant levels of infrastructure. Within the field of commercial aerospace production, the capital costs of constructing an autoclave and associated infrastructure are readily justifiable. Aerospace is an industry where low part count and exceedingly tight manufacturing tolerances are the norm. However, the fields of automotive, industrial and marine composite production have been seriously limited by the perceived need for autoclave based curing. Aside from the initial capital costs, the autoclave process requires extraordinarily expensive tooling systems, due to temperature gradients which are formed as a result of poor heat circulation, and high energy costs associated with heating the autoclave itself as well as the part within it. In many industries, the use of inert nitrogen gas also contributes to the costs associated with autoclave manufacture. The nitrogen is added to ensure that the composite remains in an inert environment should it self-ignite. In a non-aerospace environment, these high capital and running costs necessitate a high throughput as the value adding of the cure process is more difficult to justify to the customer. A high throughput is not feasible within the autoclave cure environment however as the slow ramp speed comprises between 30-70% of the overall cure time.
An alternate process involves immersion of the composite inside a bag into a bath of hot liquid to effect curing. This technique, in attempting to overcome problems associated with slow ramp-up rates, creates other problems.
For curing large components, heavy structures are needed to contain the amount of liquid needed for complete submersion. Further, heavy equipment is needed to maintain the component immersed during the cure cycle.
Further still, this method is problematic at temperatures over 100° C. The addition of boiling suppressants to water only increases the boiling point to about 107° C. So, for components requiring higher curing temperatures, oil baths are used. The oil is consumable as it adheres to the vacuum bag covering the components on removal from the bath and is expensive on a component by component basis. In addition, very few oils can be exposed to atmosphere at temperatures as high as 177° C. without oxidising, this oxidation generating corrosive elements that can damage both the vacuum bag and the equipment.
Further still, should the vacuum bag suffer a leak, the heating fluid will enter the vacuum bag, potentially damaging the component.
In the field of civil engineering, infrastructure construction and repair, the quality requirements of polymer composite structures are similar to the aerospace industry. By contrast however the size of structures and the necessity for on-site repair and manufacture have excluded the use of aerospace quality composite materials. Recently, significant numbers of repairs to bridges, buildings, dams and other concrete and steel constructions have been undertaken worldwide. These repairs have focussed on the use of wet layup or secondarily bonded carbon fibre/epoxy composite systems. These wet layup or secondarily bonded systems produce poor quality final bond strengths and unreliable repairs. In addition, the use of ambient curing epoxies generates materials with reduced properties as the structures warm up on a hot day.
The reason the infrastructure and civil engineering markets are forced to utilise poor quality polymer composites is that autoclaves and ovens cannot be taken to site or used in limited access areas. Further, the use of autoclaves on the side of a bridge is impossible due to the inability to create a pressurized seal in such a situation. This limitation in portability has prevented the use of high quality composite materials in infrastructure repair. Similarly the civil construction industry is unable to utilise polymer composite components to create large structures due to its inability to access ovens or autoclaves of sufficient size or fluid containing tanks of sufficient size. Further, the transport of large structures to site would be extremely difficult were they able to access the appropriate curing facilities. As noted above all of the methods of the prior art require the use of large and expensive equipment and are not amenable to portability.
There is a need to provide a method and apparatus for the curing of composites that provides a useful alternative to those already known in the industry.
The preceding discussion of the background to the invention is intended to facilitate an understanding of the present invention. However, it should be appreciated that the discussion is not an acknowledgement or admission that any of the material referred to was part of the common general knowledge in Australia as at the priority date of the application.